Engine turns w/ plus off but not with plugs on. " help"

I just rebuilt the motor when it was on the bench it spun fine. We installed the motor and and the engine wouldn't turn over. we pulled the plugs and the engine turns fine.
1. we replaced the battery
2. we replaced the cable going to the starter and the starter.
3. we jumped the ski with a car and it tries to turn over but it doesn't.
4. checked the grounds and all were good .
5. ran a ground to the block and nothing.
please help

first, never ever ever attach a ski to a running car. It sounds like your timing is off and you're not releasing compression. Put a 17mm socket on the supercharger nut and spin it by ratchet. If it turns over fairly easily then your starter is toast.